Study on the Influence of Temperature, Moisture and Electric Field on the Electrical Conductivity of Oil-Impregnated Pressboard

Abstract

The main insulation of converter transformers consists of transformer oil and oil-impregnated pressboard. Under operating conditions, the valve-side winding of the converter transformer is subject to DC voltage components. Therefore, studies on the characteristics of oil-impregnated pressboard conductivity are necessary. In this paper, the temperature, moisture and electric field dependency of pressboard conductivity are investigated based on a specially designed three-electrode experimental chamber, which allows for a variation in temperature ranging from 25 °C to 120 °C and an electric field strength ranging from 0 to 30 kV/mm. The experimental results show that, within the experimental conditions, the conductivity of oil-impregnated pressboard increases exponentially with increasing moisture and temperature. High moisture and temperature will increase both the carrier concentration and carrier mobility, which explains the exponential correspondence. Furthermore, the electric field dependency of the conductivity is more obvious for wet pressboard than for dry pressboard. Protons in the wet pressboard are more easily accelerated by the electric field than the impurity ions in the oil of the dry pressboard, which leads to an obvious electric field dependency of the wet pressboard conductivity.

Effect of Moisture on the Thermal Conductivity of Cellulose and Aramid Paper Impregnated with Various Dielectric Liquids

“…The increase in moisture has a number of negative consequences that are important for the transformer insulation system, but also for its other elements. First of all, reducing the resistivity of insulation materials [31] and the breakdown voltage [32,33] belong to the most important of them. As a consequence, this may lead to a decrease in the initial voltage of partial discharges [30], thus contributing to an increase in the likelihood of an unexpected transformer failure [34,35].…”

A Novel Oil-Immersed Medium Frequency Transformer for Offshore HVDC Wind Farms

“…Although in case of mineral oil, the conductivity data for high voltage DC applications are already given in literature [52,76,102,103], the measurements have been done to support a reliable design of the measuring system. The measured conductivity values, which are also summarized in Table 3-1, strongly depend on temperature, and a weak dependency to the electric stress can be observed as well.…”

Ageing of DC conduction in thermally aged oil‐impregnated pressboard

“…Oil impregnated pressboard is the primary insulating material used in converter transformers, where it has to bear significant DC stresses. It has been experimentally observed that the DC conductivity of oil-pressboard insulation increases with applied electric stress [1][2]. However, little is known about the conduction mechanism responsible for such behaviour even though the value of DC conductivity becomes very important during the calculation of electric field distribution under HVDC environment [3][4].…”

“…In these investigations it was observed that charge density increases significantly with the enhancement of external field. Some recent investigations [1][2] focused on the effect of temperature on the conductivity of oil-pressboard. However, the physical mechanisms responsible for conduction process were not investigated.…”